Metallic objects (for example catheters in vessels, platinum coils in aneurysms, dental fillings, protheses etc.) produce streak-like artifacts in X-ray computed tomography two-dimensional images, said artifacts extending across the entire image. If the artifacts are strong, the image is rendered diagnostically unusable as a result of the artifacts, since anatomical structures are covered. The same problem arises in the case of volume reconstructions from planar projection data, such as is acquired for example by means of C-arm systems and associated flat-panel detectors.
Various determination methods for determining a definitive three-dimensional reconstruction of an examination object with the aid of a plurality of two-dimensional original projection images are known from U.S. Pat. No. 7,148,903 B2. A reduction in metallic artifacts is possible using the determination methods described therein. With the known methods—irrespective of whether a provisional three-dimensional reconstruction of the examination object is determined within the scope of the determination methods or not—metal areas are determined directly in the original projection images, which metal areas are produced by metallic objects in the beam path. Data values of the original projection images which are assigned to locations that lie in the metal areas are modified. In this way modified projection images are determined. The images can be modified as a function of data values which are assigned to edge zones of the metal areas. Finally, the definitive three-dimensional reconstruction of the examination object is determined based on the modified projection images.
The approaches described in U.S. Pat. No. 7,148,903 B2 enable a substantial reduction in metal artifacts in a multiplicity of application situations. However, in many application situations—in particular when areas filled with contrast agent are present in the original projection images—it is difficult to impossible to differentiate metal and contrast agent using the approaches of U.S. Pat. No. 7,148,903 B2. It is also difficult, using the known determination methods, to detect small metal objects in the original projection images.
Iterative reconstruction methods are also known. With these reconstruction methods, streak artifacts can be reduced by including the data acquisition and the data statistics in the reconstruction algorithm. Iterative reconstruction methods are very compute-intensive, however.